Method for manufacture of tubes of a zirconium-based alloy

ABSTRACT

Method for manufacturing tubes especially for fuel elements and tubes for construction of fuel rod skeletons made of zirconium-based alloy for nuclear reactors and a tube made thereby. The tubes are ground to a surface roughness of ≦0.2 μmRa with a belt device of SiC-based belts, so-called “dressed” SiC-based belts with cotton back or SiC-cork belts. The tubes may possibly be polished after grinding with a wheel to a surface roughness &lt;0.25 μmRa.

This application is the national state application of PCT/SE99/02122,filed Nov. 19, 1999, that designated the United States of America, andwhich claims priority Swedish Application No. 9804262-5, filed on Dec.9, 1998, and which PCT application was published in English on Jun. 29,2000.

FIELD OF THE INVENTION

The present invention is directed to a method of ultrasmooth grinding oftubes of a zirconium-based alloy. The zirconium-based alloy can be usedfor cladding of fuel in thermal rectors. The method includesconventional manufacturing of tubes from the stadium of ingot to thefinal annealed tubes, ultrasmooth grinding and after thatnon-destructive and destructive testing according to conventionaltechniques. The method can be used in the manufacture of cladding tubesfor fuel rods for boiling water reactors and compressed water reactors.Additionally, the invention has application in the area of constructiontubes, such as guide thimble tubes, and instrumentation tubes for fuelrods for PWR and construction tubes for fuel rods for BWR.

BACKGROUND OF THE INVENTION

In the description of the background of the present invention thatfollows reference is made to certain structures and methods, however,such references should not necessarily be construed as an admission thatthese structures and methods qualify as prior art under the applicablestatutory provisions. Applicants reserve the right to demonstrate thatany of the referenced subject matter does not constitute prior art withregard to the present invention.

Among the alloys that are most customarily used in the manufacture ofcladding tubes for fuel rods for boiling water reactors (BWR) andcompressed water reactors (PWR), alloys such as “Zircaloy” areespecially known, amongst which Zircaloy-2 and Zircaloy-4 have thefollowing composition, in accordance with ASTMB 811:

Element Weight % Zircaloy-2 Sn 1.2-1.7 Fe 0.07-0.20 Cr 0.05-0.15 Ni0.03-0.08 Zr balance + impurities Zircaloy-4 Sn 1.2-1.7 Fe 0.18-0.24 Cr0.07-0.13 Zr balance + impurities

In addition to the above, the content of oxygen for these alloys shouldbe between 900 and 1600 ppm for cladding tubes and construction tubesfor fuel element skeletons. Furthermore, Zircaloy-2 and Zircaloy-4 arecommercially used with insignificant, but well-defined additives of Siand/or C, preferably in contents of 50-120 ppm Si and 80-270 ppm C.

These zirconium-based alloys are chosen because of their nuclearproperties, foremost being the low neutron absorption cross-section,which imparts a low ability to absorb neutrons, mechanical properties,and corrosion resistance in water and steam at high temperatures andunder radiation.

The development goes to meet longer service times for fuel rods, i.e.,to allow a higher extent of rates of burn-up. It has been proved therebyto be of importance to be able to improve the corrosion resistance tonot exceed the design related maximum oxide thickness of thezirconium-base alloyed fuel rods. Furthermore, a very important factoris to minimize the activity build-up on the fuel, i.e., on the outersurface of the fuel rods. Since the service time is extended, there arehigher demands made on the outer surface of the fuel rods, such as inreference to their condition and surface roughness. Experience has shownthat a pickled and preautoclaved surface with fine surface roughnessreduces the contribution to deposition of activity-contributing elementson the surface of the fuel rod as compared to fuel rods withconventional surface finishes with a coarser surface roughness.

It has been a widely used technique to manufacture cladding andconstruction tubes of zirconium alloys by a manufacturing process thatincludes hot working of an ingot to a bar, quenching from the β area,machining to a hollowed work piece, extrusion of the hollowed workpiecein a α-area to a tube hollow, reducing in several operations by coldrolling to substantially final dimension with a recrystallizationannealing in the α-area for each cold rolling operation. The cold rolledtube, substantially in the final dimension, will be vacuum annealed inthe α-area at a temperature in the interval of 450-650° C. andsurface-conditioned by some of the methods blasting or pickling.Alternatively, a combination of these methods can be used. Also, thetube can be surface-conditioned on the outside by mechanical treatmentin form of one, or a combination of, the methods belt grinding, beltpolishing, contact grinding and polishing.

The object of the present invention is to provide an improved method forthe manufacturing of tubes of zirconium alloys for cladding of fuel andfor construction of fuel rods for skeletons, including mechanicaltreatment of the outer surface that, after the final vacuum annealing,meet the requirements for fine surface roughness in order to minimizeactivity build-up. Certainly, a fine surface roughness (and by that, alow activity build-up) as above mentioned could be performed by whichthe outer surface of the tubes after the final vacuum annealing will bepickled and preautoclaved in high-pressure steam. However, severaldisadvantages are found with this method. At first the pickling must bedone in a mixture of nitric acid and hydrofluoric acid. This mixture isdisadvantageous in regards to the working environment and also thehandling of the consumed acid, which can contribute to an unwantedincrease of the environmental pollution. At second the autoclavingcontributes to corrosion-produced hydrogen, which will be absorbed bythe zirconium alloys. This causes the cladding and construction tubes tohave an undesirable shortening of the total operating time, since theautoclave produced hydrogen adds to the hydrogen that will be absorbedduring operation.

SUMMARY OF THE INVENTION

A method is provided for manufacturing tubes especially for fuelelements and tubes for construction of fuel rod skeletons made ofzirconium-based alloy for nuclear reactors. Tubes made by the method areground to a surface roughness of ≦0.2 μmRa with a belt device ofSiC-based belts, so-called “dressed” SiC-based belts with cotton back orSiC-cork belts. The tubes may possibly be polished after grinding with awheel to a surface roughness ≦0.25 μmRa.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

Other objects and advantages of the invention will become apparent fromthe following detailed description of preferred embodiments inconnection with the accompanying drawings in which like numeralsdesignate like elements and in which:

FIG. 1 is an exemplary belt-grinding machine with 3 belt heads

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, it has surprisingly been foundpossible to manufacture cladding tubes and construction tubes ofzirconium alloys for nuclear reactors according to a method thatcontains ultrasmooth grinding and that gives a considerably finersurface roughness on the outer surface than that for conventionalmanufactured tubes and, further, gives a surface roughness that isconsiderably finer than that represented by pickled and preautoclavedtubes. Further it has been found that tubes with an ultrasmooth groundsurface have a corrosion rate under autoclave testing in steam at 415°C. that is approximately 20% lower than for conventional manufacturedtubes. This subsequent result is entirely unexpected.

The invention provides a method of manufacturing cladding tubes andconstruction tubes of zirconium-based alloy for nuclear rectors, inwhich the zirconium-based alloy is manufactured according toconventional and known methods including melting, hot working and βquenching before extrusion, extrusion, cold rolling with intermediaterecrystallization annealing, final vacuum annealing, inner surfaceconditioning and after which the zirconium-based alloy will beultrasmooth ground instead of going through a conventional outer surfaceconditioning.

The ultrasmooth grinding will be done in a belt grinding machine,consisting of a feeder, a grinding machine and a feeding out device. Thegrinding machine consists of a number of belt heads. Each belt headconsists of two belt units, operated of one and the same engine. Thebelt unit consists of a driving wheel and a steering wheel, which runsand stretches the grinding belt and also a pressure wheel, which pressesdown the belt towards the tube while grinding. Removal of material fromthe tube-surface will be adjusted by vertically adjusting of each belthead. The belt grinding machine can be equipped with up to 5 belt heads,i.e. 10 pieces of belt units. The ultrasmooth grinding will be performedwith a sequence of belts consisting of conventional abrasive SiC-belts,conventional polishing SiC-cork belts, and also so called “dressed”SiC-based grinding belts with cotton back. FIG. 1 shows a belt-grindingmachine with 3 belt heads.

The zirconium-based alloy consists preferably of a zirconium-tin alloy.For example, the alloys can be a zirconium-tin alloy known under thetrade marks Zircaloy-2 and Zircaloy-4, that have a content of alloyingelements in the range between 1.2-1.7% for tin, 0.07-0.24% for iron,0.05-0.15% for chromium, 0-0.08% for nickel and 0.09-0.16% for oxygen,small additions of silicon and carbide, the balance zirconium andoptionally occurring inevitable impurities. Additionally, the alloy canbe of type Zircaloy-3 with 0.5-1.0% Sn, 0.3-0.6% Fe and 0-0.2% Cr andsupplements and occurring impurities corresponding to Zircaloy-2 and -4.The contents in the application refer to percentage by weight.

Preferably, Zircaloy-2 contains 1.2-1.7% Sn, 0.07-0.20% Fe, 0.05-0.15%Cr, 0.03-0.08% Ni and 0.09-0.16% O.

Preferably, Zircaloy-4 contains 1.2-1.7% Sn, 0.18-0.24% Fe, 0.07-0.13%Cr, 0.09-0.16% O, no Ni.

The invention will be explained in detail by describing one of theembodiments.

Cladding tubes of Zircaloy-2 were manufactured according to theconventional methods regarding to all process steps from melting of theingot to the final annealing at 575° C. After the conventional surfaceconditioning of the inner surface, the product is exposed to ultrasmoothgrinding at which the product will be ground in a grinding machine,consisting of 6 belt units. In order to obtain the optimal surfaceroughness the tubes are exposed to two grinding passages in the machinewith the following sequence of belts:

Pass 1

2 × 320 grit, conventional SiC-belts 2 × 400 grit, conventionalSiC-belts 2 × 600 grit, “dressed” SiC-based belts with cotton back

Pass 2

2 × 800 grit, “dressed” SiC-based belts with cotton back 2 × 1000 grit,conventional SiC-cork belts 2 × 1200 grit, conventional SiC-cork belts

After the ultrasmooth grinding the tubes were split into two lots. Oneof the lots was exposed to another surface treatment in the form ofpolishing with a 800-grit wheel.

The surface roughness of the tubes was measured. For the tubemanufactured according to the present invention, it was observed thatthe surface roughness after ultrasmooth grinding is ≦0.2 μmRa and thesurface roughness after ultrasmooth grinding and polishing with 800-gritwheel is ≦0.25 μmRa. The surface roughness for the conventionalmanufactured tubes is in the range 0.3-0.5 μmRa. For conventionalmanufactured tubes exposed to an outside pickling as well as autoclavetreatment the surface roughness is ≦0.25 μmRa.

Under corrosion testing in steam at 415° C. for 90 days—a test thatsimulates the conditions regarding the resistance against generalcorrosion during reactor operations—tubes, which were manufacturedaccording to the present invention, exhibit a better resistance againstgeneral corrosion than conventional manufactured tubes. The conventionalmanufactured tubes have proven good resistance against general corrosionand were included in the trial for comparison. Test results for thecorrosion rate measured between the exposing time of 60 and 90 daysexpressed in mg/dm²/day, is as follows:

Ultrasmooth ground tubes: 0.53 Ultrasmooth ground plus polished tubes:0.60 Conventional manufactured tubes: 0.67

The ultrasmooth grinding leads to a 20% decrease of the corrosion rate20% and ultrasmooth grinding plus polishing leads to a 10% decrease,compared with conventional manufactured tubes.

Although the present invention has been described in connection withexemplary embodiments thereof, it will be appreciated by those skilledin the art that additions, deletions, modifications, and substitutionsnot specifically described may be made without departing from the spiritand scope of the invention as defined in the appended claims.

What is claimed is:
 1. A method of grinding a zirconium-based alloy tubeto provide a surface roughness of the tube of ≦0.2 μmRa, the methodcomprising: (i) subjecting the tube to a first grinding pass through afirst sequence of Si—C-based abrasive belts, the belts having a gritsize of 320-600; and (ii) subjecting the tube to a second subsequentgrinding pass through a second sequence of Si—C-based abrasive beltshaving a grit size of 800-1200.
 2. The method according to claim 1,wherein steps (i) and (ii) are performed with a grinding machine with 6to 10 belt units.
 3. The method of claim 1, wherein the Si—C-based beltscomprise at least one of: Si—C-cork belts or SiC-based belts with acotton back.
 4. The method according to claim 1, further comprisingpolishing the surface with a wheel to a surface roughness of <0.2 μmRa.5. The method according to claim 4, wherein the polishing is performedwith a wheel with grit size 600-1200.
 6. The method according to claim1, wherein steps (i) and (ii) are performed before or after surfaceconditioning of an inner surface of the tube.